Same titles in the same playlist

This is my third try to post in the correct forum, sorry.

Sorry for my poor English but why is it even possible to add the same song to the same Playlist over and over again without any notice. In Spotify or any other app I get a pop up, that this Song already exists in that chosen playlist. How should I know if I added a song to a specific playlist weeks ago? This sucks. If Im just dumb, please let me know. Thanks.